Asbestos is a range of complex silicates. It is an extremely refractory material and is widely used for high temperature insulation. It is not thought to be an easy material to convert into a glass, indeed asbestos entering a glass furnace accidentally has been known to produce solid inclusions in the finished glass. But since asbestos is a toxic material some means of converting waste asbestos into an inert substance is required.
The use of inter alia asbestos in glass manufacture was proposed by Crossley in U.S. No. Re. 15727 of U.S. Pat. No. 1,394,973. He proposed to heat vitreous material and mica or asbestos to a temperature above the melting point of the glass but below the decomposition temperature of the asbestos so that the physical and chemical characteristics of the mica or asbestos were preserved, and temperatures of 650.degree.-900.degree. C. were employed. But insofar as the asbestos was not destroyed and did not become part of the glass matrix, it retained its toxic properties, and glass derived from asbestos according to Crossley's process would not now be regarded as safe to discharge on a waste tip. The vitrification of asbestos was also taught by Santt in French patent specification No. 2398704 but he considered incorrectly that asbestos is destroyed at temperatures above 600.degree. C. and relied on complex iron-containing mixtures to form a glass or frit.